This invention relates to a wireless receiver and, more particularly, relates to a wireless receiver for improving intelligibility of reception signals and a reception method therefor.
A tone quality switching function of a speaker in a wireless receiver, for example, is one of conventional means for improving intelligibility of reception signals in the wireless receiver. FIG. 1 of the accompanying drawings is a block diagram showing a structural example of a wireless receiver according to the prior art. This drawing shows in detail the construction of a speech power amplification circuit equipped with a speaker tone quality switching function.
As shown in FIG. 1, the wireless receiver includes a reception unit having an antenna 50, a front end portion 51, a first intermediate frequency portion 52, a second intermediate frequency portion 53, a detection circuit 54 and a speech signal processing circuit 55, a speech power amplification circuit 20 coupled to the reception unit and a speaker 2 for reproducing the speech signal form the speech power amplification circuit 20.
The radio wave transmitted from the not-shown partner side wireless transmitter is received by the front end portion 51 through the antenna 50 as the wireless frequency signal (high-frequency signal). The wireless frequency signal received by the front end portion 51 is down-converted to the first intermediate frequency signal by the first intermediate frequency portion 52, and then further down-converted to the second intermediate frequency signal by the second intermediate frequency portion 53.
The output signal from the second intermediate frequency portion 53 is demodulated by the detection circuit 54 and supplied to the speech signal processing circuit 55.
The speech signal processing circuit 55 limits the frequency band of the demodulated signal from the detection circuit 54 to the speech band and amplifies with a predetermined gain thereby to output as a base band signal (speech band signal) to the speech power amplification circuit 20. The speech power amplification circuit 20 includes a tone quality control unit 22 for controlling tone quality of a base band signal from the speech signal processing circuit 55 and a speech power amplifier 14 for amplifying the speech signal controlled by the tone quality control unit 22. The tone quality control unit 22 includes a switch 13, a central processing unit (CPU) 7 such as a microcomputer, a resistor 9, capacitors 10 and 11 and an analog switch 12.
The speech band signal outputted from the speech signal processing circuit 55 is amplified by the speech power amplifier 14 and is outputted as a reception speech from the speaker 2. The analog switch 12 is controlled to an ON state when the control input from the central processing unit 7 is at an "H" (high level) and to an OFF state when the control input is at an "L" (low level), for example.
The circuit including this speech power amplifier 14 forms a high-pass filter (HPF). Because the analog switch 12 is an ON state at the normal time, the cut-off frequency of the HPF is determined by the following equation (1): EQU fc=1/2{2.pi.R(C.sub.1 +C.sub.2)} (1)
Here, R is a resistance value of the resistor 9 and C.sub.1 and C.sub.2 are the capacitance values of the capacitors 10 and 11, respectively.
When the switch 13 is turned ON, the tone quality switching function is made ON (effective) and the central processing unit 7 outputs a control signal of the "L". Therefore, the analog switch 12 is turned OFF and the cut-off frequency of the HPF is determined by the following equation (2):
fc=1/2.pi.RC (2)
Therefore, the cut-off frequency becomes high when the tone quality switching function is turned ON or made effective. FIG. 2 shows a gain-v-frequency characteristic curve at this time. FIG. 3 shows an NQ (Noise Quieting) curve of the wireless receiver by using a saturation value of an S (Signal) curve (at the time of reception of a 1 kHz standard modulated wave) as a reference level. The frequency characteristics at the time of tone quality switching function ON will be compared with the frequency characteristics at the normal time (tone quality switching function OFF) with reference to the characteristic example shown in FIG. 3. It can be appreciated that the noise output level decreases at a reception input field intensity of about -130 dBm or below at the time of tone quality switching function ON in comparison with the normal time.
In the practical operation of the wireless receiver, however, weak field signals below about -130 dBm are those which are hardly intelligible, and no practical improvement can be attained in intelligibility even when the noise output level is reduced to some extents by turning ON the conventional tone quality switching function.
On the other hand, the reception input field intensity, at which the noise output level increases and intelligibility starts deteriorating drastically is around about -120 dBm on the NQ curve shown in FIG. 3 (at the normal time). Referring to FIG. 3, a remarkable decrease of the noise output level cannot be observed in this area near about -120 dBm even when the tone quality switching function is turned ON. Therefore, even when a user of the conventional wireless receiver recognizes deterioration of intelligibility and turns ON the tone quality switching function during the practical operation, a substantial improvement in intelligibility cannot be acquired easily, and the tone quality switching function often remains at the level of the taste of listening of the user of the wireless set.